An internal combustion engine, particularly of the two-cycle variety, is often used to power hand-held lawn and garden equipment, such as flexible line trimmers, blowers and chain saws. The noise of the internal combustion engine is not only a nuisance to the operator of the tool and to others in the vicinity, but it can also, due to the operator's very close proximity to the motor, tend to harm the operator's hearing. However, noise reduction must often be achieved at the expense of engine performance and compactness, simplicity, reliability, ease of manufacture and cost of the muffler.
Mufflers reduce the sound level of the exhaust from the engine typically by dissipating the pressure and velocity of the exhaust gas before it is released to the atmosphere. However, mufflers deleteriously affect engine performance. Dissipating pressure and velocity tends to create undesirable back pressure on the exhaust flow from the engine. Too much back pressure impedes scavenging of the engine's cylinder, reducing efficiency and power. More sophisticated and complex structures tend to be required to dissipated the noise without creating too much back pressure.
However, the same aspects that make a two-cycle engine desirable for powering handheld lawn and garden tools--compactness, simplicity, and low cost manufacture, operation and maintenance--are also desirable for a muffler. As the cost of the entire lawn and garden product is not very expensive, the muffler must be kept very inexpensive to manufacture. It also must remain compact and light-weight, as well as reliable and easily maintained. Consequently, noise reduction structures tend to be kept relatively simple, at the expense of noise reduction and engine performance.
More complex structures also present greater reliability problems. Consumers expect lawn and garden equipment to function indefinitely, without maintenance. Mufflers, mounted directly to engines for compactness, are subject to significant vibrations from operation of the engine. They are also generally subject hostile conditions of operation and storage. Mufflers for this equipment therefore face significant reliability demands. An example of this reliability problem is an attenuating tube. An attenuating tube essentially tunes the muffler and produces a flow of exhaust gas at its output at a pressure that is as close as possible to atmospheric pressure and is substantially constant. The attenuating tube is manufactured by rolling a sheet of metal into a tube, cutting the tube to the desired length, punching a hole in the exterior shell of the muffler to accommodate the tube, inserting the tube in the hole, brazing the tube in place for a good seal and then retaining the loose end of the tube by use of other components within the muffler. Not only is it expensive to manufacture a muffler with an attenuating tube, the vibrations of the engine place the braze under great stress, and thus tend to eventually shake the tube loose.
Finally, muffler designers must also contend with the U.S. Department of Agriculture requirement that the exhaust from multi-position engines used in U.S. National Parks be vented to the atmosphere through a spark arrestor. As gas-powered lawn and garden tools, such as chain saws, are often used in U.S. National Parks, mufflers are often fitted with or are made capable of being fitted with spark arrestor. To save costs, this typically involves simply placing the spark arrestor screen between the muffler and the exhaust output port of the engine or at the output of the muffler. The positioning of the spark arrester screen becomes very important with respect to its longevity and efficiency. If the screen is placed to close to the exhaust inlet port, the screen can be degraded prematurely by the intense heat. On the other hand, if the screen is positioned too far away from the exhaust inlet port, the carbon tends to build up on the screen because the exhaust gas is too cool. The carbon build-up clogs the openings of the screen, causing an undesirable amount of back pressure that tarnishes engine performance.
U.S. Pat. Nos. 3,638,756, 4,415,059, 4,759,423, 4,765,437, 4,821,840, 4,836,330, 4,901,815, 4,924,568, 9,958,701 and 5,004,069 show various types of mufflers for large-displacement, four-cycle internal combustion engines using assembled multiple stamp-formed members that tend to simplify manufacture and assembly. However, such mufflers are unsuitable for use with two-cycle internal combustion engines used on powered lawn and garden equipment, where compactness, lightweight, simplicity, special concerns of the two-cycle engine and special requirements of the USDA are premium concerns that weigh heavily on noise reduction performance.
Prior art mufflers suitable for use with gas-powered lawn and garden tools have not offered the most desirable combination of noise reduction, engine performance, simplicity, cost and reliability. Therefore, it is one object of the invention to provide good noise reduction performance while maintaining good engine performance by using a design having reliability and low cost manufacture.